Radio frequency identification (RFID) systems typically use one or more reader antennae to send radio frequency (RF) signals to items tagged with RFID tags. The use of such RFID tags to identify an item or person is well known in the art. In response to the radio frequency (RF) signals from a reader antenna, the RFID tags, when excited, produce a disturbance in the magnetic field (or electric field) that is detected by the reader antenna. Typically, such tags are passive tags that are excited or resonate in response to the RF signal from a reader antenna when the tags are within the detection range of the reader antenna.
The detection range of the RFID systems is typically limited by signal strength to short ranges, for example, frequently less than about one foot for 13.56 MHz systems. Therefore, portable reader units may be moved past a group of tagged items in order to detect all the tagged items, particularly where the tagged items are stored in a space significantly greater than the detection range of a stationary or fixed single reader antenna. Alternately, a large reader antenna with sufficient power and range to detect a larger number of tagged items may be used. However, such an antenna may be unwieldy and may increase the range of the radiated power beyond allowable limits. Furthermore, these reader antennae are often located in stores or other locations where space is at a premium and it is expensive and inconvenient to use such large reader antennae. In another possible solution, multiple small antennae may be used but such a configuration may be awkward to set up when space is at a premium and when wiring is preferred or required to be hidden.
Current RFID reader antennas are designed so that a maximum read range may be maintained between the antenna and associated tags, without running afoul of FCC limitations on radiated emissions. Often times, when tagged items are stacked, the read range of an antenna is impeded due to “masking” that occurs through the stacking. As a result, the masking limits the number of tags that an antenna may read through, and consequently affects the number of products that may be read. Furthermore, due to FCC limitations on radiated emissions, the reader antenna sizes cannot be adjusted to resolve such problems.
Resonant loop reader antenna systems are currently utilized in RFID applications, where numerous reader antennas are connected to a single reader. Each reader antenna may have its own tuning circuit that is used to match to the systems characteristic impedance. Multiple antennae (or components) may require the use of multiple transmission cables to connect a reader unit to the multiple antennae and/or to individually control the multiple antennae when they are all connected by a single transmission cable to the reader unit.
RFID applications incorporating random placement of a product may result in formation of “dead zones” for orientations in which the tag and reader antenna are in orthogonal planes. Dead zones are areas (dependent upon tag/reader antenna orientation) in which the level of coupling between the reader antenna and tag is not adequate for the system to perform a successful read of the tag. Thus, products placed in dead zones may not be detected resulting in potentially inaccurate tracking of tagged products.